The quality of chemical fibers refers to the indicators that determine the service value of fiber products.
Physical‑property indicators: including fiber length, fineness, density, luster, moisture absorption, thermal properties, electrical properties, etc.
Mechanical‑property indicators: including breaking strength, breaking elongation, initial modulus, resilience, repeated‑deformation resistance, etc.
Stability‑property indicators: including stability against high and low temperatures, stability against light and atmosphere, stability against chemical reagents, stability against microbial action, etc.
Processing‑property indicators: including fiber cohesion, static‑charging tendency, dyeability, etc.
Additional quality indicators for staple fibers: including fiber length, crimp degree, fiber defects, etc.
1. Fineness
Fineness denotes the thickness of fibers, divided into direct indicators and indirect indicators. Direct indicators are generally expressed by fiber diameter and cross‑sectional area. Since fiber cross‑sections are irregular and difficult to measure, direct indicators are seldom used for thickness description; indirect indicators are therefore widely adopted. Indirect indicators are defined based on fiber mass or length, namely fiber mass at fixed length (fixed‑length system) or fiber length at fixed mass (fixed‑weight system). In the chemical‑fiber industry, fineness is usually represented by linear density, i.e. mass per unit fiber length. Three common expressions are shown below:
1.1 Tex or dtex
Tex and decitex (dtex) are SI units. Tex: mass in grams of a 1000‑meter‑long fiber. One‑tenth of a tex is decitex (dtex). As fibers are fine, tex values are small, so decitex is more commonly used for fiber fineness.
For fibers of the same type (constant density), the lower the tex number, the finer the single fiber, the softer the hand‑feel, the milder the luster, and the easier for deformation and processing.
1.2 Denier
Denier is defined as the mass in grams of a 9000‑meter‑long fiber. For fibers of the same type, lower denier means finer single fiber. Denier is a non‑legal unit of linear density. 1 den = 9 tex.
1.3 Metric Count (Nm)
Metric count, short for metric number, refers to fiber length in meters per gram of fiber mass. For fibers of the same type, higher count indicates finer fibers. Metric count is a non‑legal unit of linear density.
Mutual conversion relationships:
Denier × Metric Count = 9000
Tex × Metric Count = 1000
Denier = 9 × Tex
dtex = 10 × Tex
2. Density
Fiber density means the mass per unit volume of fiber, unit: g/cm³. Fiber densities vary among varieties. Among major chemical fibers, polypropylene fiber has the lowest density, while viscose fiber has the highest density.
3. Moisture Absorption
Fiber moisture absorption refers to water‑absorbing capacity under standard conditions (20 ℃, 65 % relative humidity). Two indicators are applied:
Moisture Regain: ratio of water mass contained in fiber to oven‑dry fiber mass.
‑
Moisture Content: ratio of water mass contained in fiber to wet fiber mass.
Moisture Content=
Mass of undried sample
Mass of water in sample
×100%
Moisture absorption differs greatly among fiber types, and also varies with ambient temperature and humidity. For weighing and pricing purposes, a unified specified moisture regain for textiles is defined as official moisture regain.
Natural fibers and regenerated fibers have high moisture regain; synthetic fibers show low moisture regain. Among them, polypropylene fiber, polyvinyl chloride fiber and polyethylene fiber have zero moisture regain.
Moisture absorption affects processing and service performance. Fibers with good moisture absorption reduce friction and static electricity and provide wearing comfort. For synthetic fibers with poor moisture absorption, modification methods can improve their hygroscopicity.
4. Tensile Properties
Fibers are subjected to tension, bending, compression, friction and torsion in service, resulting in various deformations. Tension is the main external force applied to chemical fibers; bending performance is also related to tensile properties. Therefore, tensile properties are the most important mechanical properties of fibers. Three key indicators are breaking strength, breaking elongation and initial modulus.
4.1 Breaking Strength
Relative strength is commonly used for chemical‑fiber breaking strength: the ratio of maximum load borne by fiber until rupture under continuously‑increasing load to fiber linear density. Units: N/tex, cN/tex.
Breaking strength is a critical quality indicator. High breaking strength reduces end breakage and roller wrapping during processing, and improves yarn and fabric durability. Excessively high strength, however, increases fiber rigidity and stiffens hand‑feel.
Strength measured under dry condition is dry strength; strength measured under wet condition is wet strength. Fibers with high moisture regain have lower wet strength than dry strength. Most synthetic fibers have low moisture regain, so their wet strength is close or equal to dry strength.
4.2 Breaking Elongation
Breaking elongation is the relative elongation at break, i.e. the percentage increase of fiber length at rupture versus original length.
Y=
L
0
L−L
0
×100%
Where:
L
0
— original fiber length
L
— fiber length at break
Breaking elongation reflects fiber toughness. For apparel filament yarns, higher elongation brings softer hand‑feel and fewer broken filaments and end breaks in downstream processing; yet excessive elongation causes fabric deformation. For industrial‑grade filaments, lower elongation helps finished products resist deformation.
4.3 Initial Modulus
Initial modulus, also elastic modulus, is the stress required to stretch a fiber to 1 % of its original length.
It characterizes fiber resistance to small deformation, and reflects fabric stiffness under minor tensile or bending loads in apparel applications. Higher initial modulus means better anti‑deformation performance. Among major synthetic fibers, polyester fiber has the highest initial modulus, followed by acrylic fiber; polyamide fiber shows a low value. Accordingly, polyester fabrics are stiff‑textured and wrinkle‑resistant, whereas polyamide fabrics wrinkle easily with poor shape retention.
5. Resilience
Resilience (elastic recovery) is the capacity of a material to restore its original shape after removal of external tensile or compressive load. Deformation under load consists of three components: instantaneous elastic deformation, high‑elastic deformation and plastic deformation. These three deformations develop simultaneously rather than sequentially, only differing in rate.
After load removal, recoverable instantaneous elastic deformation and part of high‑elastic deformation with short relaxation time (rapid elastic recovery) rebound rapidly. Remaining deformation includes high‑elastic deformation with long relaxation time (slow elastic recovery) and irreversible plastic deformation. Smaller residual deformation indicates better fiber resilience.
6. Evenness Irregularity
Evenness irregularity describes filament evenness, expressed as CV value (coefficient of variation) or Uster %. This indicator is especially critical for oriented yarns and drawn yarns. Uneven filament evenness easily causes broken filaments and uneven dyeing in downstream processing.
7. Crimp Degree (mainly for staple fibers)
Crimp is imparted to fibers via chemical, physical or mechanical treatment.
Purpose of crimping: improve fiber cohesion, enhance bulkiness and elasticity, so as to achieve favorable appearance and warmth retention of fabrics.
Indicators for characterizing staple‑fiber crimp are listed below.
8. Boiling‑Water Shrinkage
Boiling‑water shrinkage is the ratio of fiber length after boiling in hot water for 30 minutes to original length.
It reflects fiber heat‑setting degree and dimensional stability. Lower boiling‑water shrinkage means better structural stability. Fibers maintain dimensions and resist deformation under hot‑wet treatments such as dyeing and washing, with favorable physical‑mechanical and dyeing properties. Boiling‑water shrinkage is controlled by heat‑setting process parameters.
9. Combustion Performance
Combustion performance describes how easily fibers burn in air. The internationally‑adopted test method is Limit Oxygen Index (LOI). LOI is the minimum volume percentage of oxygen in nitrogen‑oxygen mixed atmosphere that enables a flame‑ignited fiber to keep burning after leaving the ignition source.
Fiber classification by LOI:
Combustible / Readily‑flammable fibers: LOI ＜ 21 %
Flame‑retardant fibers: LOI ＞ 21 %
Fire‑resistant fibers: LOI ＞ 26 %
Most common chemical fibers have LOI below 21 % and belong to combustible or readily‑flammable fibers.
Extensive domestic‑overseas research has been carried out on flame‑retardant modification of chemical fibers. Main approaches include copolymerization, blending and surface treatment. Organic phosphorus compounds, organic halogen compounds or their combinations are introduced into fibers or fabrics.
10. Abrasion Resistance
Fibers and their products wear due to continuous friction during processing and service. Abrasion resistance refers to fiber capacity to withstand abrasive wear.
Abrasion resistance is closely related to textile durability, and is a key service‑performance indicator for apparel fabrics. It is affected by fiber macromolecular structure, supramolecular structure, breaking elongation, elasticity and other factors.
Abrasion‑resistance sequence of common fibers:
Polyamide fiber ＞ Polypropylene fiber ＞ Polyvinyl alcohol fiber ＞ Polyethylene fiber ＞ Polyester fiber ＞ Acrylic fiber ＞ Polyvinyl chloride fiber ＞ Wool noil ＞ Cotton ＞ Ramie ＞ Polynosic fiber ＞ Cuprammonium fiber ＞ Viscose fiber ＞ Acetate fiber ＞ Glass fiber
11. Light Resistance and Weather Stability
Stability against sunlight and atmosphere, also named weather resistance, measures fiber capability to resist property changes induced by climatic conditions.
Light resistance denotes retention of mechanical properties upon light irradiation. Atmospheric stability refers to resistance to degradation, photo‑oxidation and discoloration under long‑term combined effects of light, oxygen, heat and moisture.
Light resistance of chemical fibers is associated with macromolecular segment composition, main‑chain bonds, cross‑link formation, molecular group vibration‑transition energy, aggregation‑state structure, as well as light intensity, irradiation duration and wavelength.
Property changes caused by climate mainly stem from sunlight and atmospheric oxygen. Improving light resistance and atmospheric stability enhances photo‑stability and oxidation stability.
Post time: Aug-31-2026